In recent years, the use of composite materials is increasing in aerospace, automobile and other industries for the purpose of reducing product weight, etc. A laser ultrasonic flaw detection apparatus is employed as a method of non-destructively evaluating the structural integrity of such composite materials, etc.
The principle of this laser ultrasonic flaw detection apparatus will be briefly described. First, when a first laser beam is focused onto a surface of an inspection object, ultrasonic vibrations are generated due to the thermoelastic effect. Specifically, the surface of the inspection object is heated by the laser beam; the volume of the inspection object expands along with this increase in temperature, thus generating stress; and ultrasonic vibrations are generated by this stress.
The ultrasonic vibrations propagate from the surface of the inspection object to the interior thereof, and, if there is a defect inside the inspection object, the ultrasonic vibrations are reflected at this defect site thus vibrating the surface of the inspection object again. When a second laser beam is focused onto this vibrating surface of the inspection object, the second laser beam is reflected at the surface of the inspection object, and the ultrasonic vibrations reflected at the defect site inside the inspection object are superimposed on the reflected light. Therefore, flaw detection can be performed for a defect inside an inspection object by extracting the ultrasonic vibrations by guiding the reflected light of the second laser beam to a laser interferometer or the like. At this time, in order to guide only the second laser beam to the laser interferometer without guiding the first laser beam to the laser interferometer, the wavelength of the first laser beam is made different from the wavelength of the second laser beam, and only the reflected light of the first laser beam is removed with a wavelength filter (see Patent Literatures 1 to 3).